The present invention relates to a method and a system in a paper web finishing machine or equivalent, the method and system being defined in the preambles of the independent claims presented below.
The present invention relates particularly to a method and a system in a paper web finishing machine or a corresponding machine, such as in a paper roll unwinding machine, in a calender or in a machine stack, so that the air which surrounds the machine and which contains impurities and/or steam can be discharged in a controlled manner.
There is formed fine dust around finishing machines, for instance in paper roll unwinding machines, which dust is easily carried to other parts of the machine room by the upwards flowing air flows generated by the heat of the machines and/or paper. The impurities reduce the ambient air quality and may cause problems in machines and equipment, and therefore they should be prevented from entering the air in the machine room.
In order to solve this problem the U.S. Pat. No. 5,635,031 proposes to generate above the machine an air curtain which extends from the tending side of the machine to the drive side of the machine so that the air curtain sucks dusty air surrounding the machine to the drive side of the machine, where the dusty air and the air generating the curtain are discharged through an outlet opening arranged at the floor level. Additional air jets are simultaneously blown on the drive side in order to control the air curtain. The additional air jets, which partly have the same direction as the air curtain, are directed from the top obliquely downwards towards the air curtain. Their purpose is to turn the tail of the air curtain downwards at the machine's drive side.
Now it has turned out in this previously known solution that in addition to the dusty air a very large quantity of clean air is carried to the drive side of the machine, from where it must be removed.
In the previously known solution the first air curtain drags along both dusty air from beneath the curtain and clean air from the machine room above the air curtain. The air jets blown from the drive side towards the air curtain increase the quantity of air to be circulated, and in addition they drag along air from the machine room towards the discharge opening for dirty air arranged on the drive side. The removal of all this dusty air and clean air from the drive side requires large-sized ducts and a relatively high fan energy consumption. Of course circulation of a large quantity of clean air should be avoided, also because in addition to voluminous ducts and fans a large quantity of air require large equipment for their cleaning before they are returned to the machine room or discharged into the environment. It would be conceivable to reduce the air flows by decreasing the velocity and volume flow of the air jets generating the air curtain, but then there is nevertheless a risk in that the air curtain when heated begins to flow upwards and not in a controlled path over the machine towards the discharge opening, and thus the air curtain does not appropriately prevent the dusty air from spreading into the machine room.
If the air discharge opening or openings are not dimensioned to be sufficiently large, then a part of the air flowing downwards on the drive side will turn away from the discharge opening and under the machine, raising dust from the floor into the air and thus increasing the quantity of impurities in the air. An air flow beneath the machine has been found to be particularly inconvenient in an unwinding machine for thin paper, such as tissue paper, where the paper web is moved forwards close to the floor level. An air flow directed beneath the machine causes flutter in the tissue paper, which in turn causes runnability problems in the machine. One solution to this problem would be to dimension the discharge ducts, the fans and the air cleaning equipment substantially bigger, so that the total quantity of air can be removed through the discharge openings. However, this would increase the costs and would also require more space.
Particularly steam emissions cause inconvenience in calenders. The steam emissions increase when speeds in paper machines and thus also in on-line calenders increase, because the quantity of steam brought to the paper surface and thus leaking from the calender is constant per paper square meter. An on-line calender leaks steam in a quantity which may be even more than the double compared to traditional calenders. Due to this the humidity in the machine room increases substantially. In order to remove the humidity it is necessary to increase the ventilation, which is expensive.
The object of the present invention is therefore to provide an improved method and a system where the above mentioned problems are minimised in a paper web finishing machine or equivalent.
The object is particularly to provide a method and a system which avoid the recirculation of an excessive quantity of air.
A further object is to provide a method and a system which avoid inconvenient air flows in the vicinity of the machine.
An object is also to provide a method and a system which can reduce the leaking of steam, for instance from a calender into the air of the machine room.
An object is thus also to provide a method and a system with which the air curtain arranged above the machine can be guided in a controlled manner.
In order to attain the above mentioned objects the method and the system according to the invention are characterised in what is defined in the characterising clauses of the independent claims presented below.
Now it was surprisingly found that the above mentioned problems for instance in a paper roll unwinding machine can be solved by arranging a second shorter air curtain above the first air curtain generated over the machine from the tending side to the drive side, so that the direction of the second air curtain is mainly opposite to that of the first air curtain. It was found that a second air curtain with an opposite direction will in a clearly better way than previously prevent the first air curtain from rising upwards at its tail end, even though the second air curtain would have a very low velocity, even &lt;0.5 m/s, and a high tendency to rise upwards. At the same time it was found that the upper air curtain prevents clean air from the environment from being dragged along the tail end of the first air curtain. The clean air of the upper air curtain is directed, differently than previously, into a different direction than the air of the first air curtain, and thus the mainly clean air of the second air curtain does not substantially mix with air which contains impurities, and thus it is not necessary to recycle the clean air via air ducts and air cleaning equipment back to the machine room.
The first air curtain is typically generated with a blowing device, such as blow beam, which is arranged above the machine in front of the tending side, and whose length is generally about the same as that of the machine, whereby the blowing device in a solution according to the invention is provided with
a slot nozzle with a length mainly equal to that of the machine and having a slot width b.sub.1 &lt;15 mm, or PA1 a plurality of slot and/or orifice nozzles arranged one after another in the machine direction and having slot widths b.sub.1 &lt;15 mm and an orifice diameter between 10 and 40 mm, or PA1 a plurality of separate blow nozzles arranged one after another in the machine direction and having a nozzle diameter between 20 and 40 mm.
Thus, instead of a single long blowing device, it is also possible to mount a plurality of shorter blowing devices one after another in the machine, so that each blowing device covers a desired area of the machine. The length of a paper roll unwinding machine is typically about 10 to 40 m, whereby a plurality of blowing devices can be advantageously installed one after another above the tending side of the machine covering this distance, so that they can provide a suitable air curtain to cover a desired area of the machine. Correspondingly, in a solution according to the invention, a second blowing device or second blowing devices is/are arranged on the other side of the machine, on the drive side, and above the level of the first air curtain, for generating a second air curtain which covers a part of the first air curtain on the drive side of the machine. The second curtain covers according to the present invention about 20-50% of the length of the first curtain.
The second air curtain is advantageously generated about 1 to 2 m above the first air curtain so that it travels in a mainly horizontal plane, or in a plane which deviates at most 10.degree. from the plane of the first air curtain.
The first air curtain is advantageously generated about 1 to 2 m above the machine and blown generally horizontally so that it extends from the first side of the machine to its second side, whereby the length of the air curtain in a typical application is about 8 to 10 m. When required the first air curtain can be blown slightly obliquely, or so that it deviates from the horizontal plane, from the first side of the machine to the second side. The second air curtain is blown in the opposite direction, i.e. so that it travels in a plane mainly parallel to, or almost parallel to the first curtain, but in the opposite direction.
The first air curtain (curtains) is (are) generated by air jets having a velocity and volume flow being such that the kinetic energy of the air jets is sufficient in order to carry the air curtain over the machine. However, in a solution according to the invention the air jets are dimensioned so that the jet velocities in the direction of the blow are at the second side of the machine almost zero, or at least &lt;0.5 m/s, whereby the air jets can be easily guided downwards on the drive side into the desired direction towards the discharge opening, due to the effect of the second air curtain and the suction of the discharge opening. In this way it is possible to avoid the problem related to an air curtain arriving at a high velocity whereby the air jet (jets) is dispersed as it strikes with a strong force the wall defining the drive side and thereby flows in an uncontrolled way carrying away dusty air, also upwards into the clean air of the machine room. Then also an air flow along the floor is avoided, and its disturbing effect on the web flow, affecting the runnability of the machine, is avoided.
Above reference was mainly made to an air curtain crossing the machine, or travelling at a right angle to the machine direction. An air curtain of this kind typically travels horizontally, even if it, of course, can, if desired, travel obliquely upwards or downwards. In other applications, the air curtain can, however, be arranged to travel in the machine direction, in the web direction or against the web direction. In these other applications the air curtain typically travels perpendicularly or obliquely upwards. An upward travelling air curtain of this kind can be arranged for instance in connection with a calender to provide an air curtain in front of the stack of calender rolls, to prevent steam from freely escaping into the machine room. The air curtain is generated by blowing air jets from a blowing device, which is arranged at a small distance from the lowermost calender rolls and arranged to blow air jets obliquely upwards towards the upper part of the calender. The air curtain guides the steam escaping from around the calender rolls towards the upper part of the calender, where, according to the invention, a suction box or a corresponding device may be installed in order to guide in a controlled manner the tail end of the air curtain and remove air which contains steam.
A suction box or equivalent extending over the web at the top of the calender may be arranged to remove air from two directions, both from the inlet side and the outlet side of the calender. Further, when desired, the suction box or equivalent can be provided with projections in the machine direction, for instance above the edges of the web, whereby suction can be arranged at those places where the steam rises upwards in the calender.
With the aid of an air curtain and/or a suction box it is possible to turn the direction of the steam rising upwards from the calender so that the steam travels towards the suction slot of the suction box and is sucked into the box.